1. Field of the Invention
The present invention relates to an optical information recording medium and a method of manufacturing the same, and in particular to an optical information recording medium for performing recording by a laser beam (blue laser) with a wavelength of 350 to 500 nm and a method of manufacturing the same.
2. Description of the Related Art
Conventionally, in addition to an optical information recording medium for performing recording and reproduction of optical information using a laser beam, that is, a CD using red laser light with a wavelength of 750 to 830 nm (e.g., around 780 nm) or a DVD using short wavelength red laser light with a wavelength of 640 to 680 nm (e.g., 650 to 665 nm), a blue laser disc (BD or HD-DVD), which can make it possible to record and reproduce high-density optical information at high speed by using a blue laser with a wavelength as short as 350 to 500 nm (e.g., around 405 nm), has been actively developed (see, for example, JP-A-2003-331465).
As one of standards concerning the blue laser disc (BD or HD-DVD), it is provided that BCA is performed.
FIG. 14 is a schematic plan view of an optical information recording medium 1 using the blue laser disc (in particular, HD-DVD) FIG. 15 is a schematic sectional view thereof.
The optical information recording medium 1 includes, in particular, as shown in FIG. 15, a translucent substrate 2, an optical recording layer 3 (a light-absorbing layer) formed on the substrate 2, a light reflection layer 4 formed on the optical recording layer 3, a protective layer 5 (an adhesive layer) formed on the light reflection layer 4, and a dummy substrate 6 stacked on an upper layer on the protective layer 5 at a predetermined thickness. The optical information recording medium 1 is formed at a predetermined thickness required by standards.
Spiral pre-grooves 7 are formed in the substrate 2. Portions other than the pre-grooves 7, that is, lands 8 are located on the left and the right of the pre-groove 7.
A laser beam 9 is irradiated from the substrate 2 side, whereby the optical recording layer 3 absorbs energy of the laser beam 9 to generate heat. Recording pits 10 are formed by thermal decomposition of the optical recording layer 3 to make it possible to record information in the optical recording layer 3.
As the translucent substrate 2, a material with high transparency with a refractive index with respect to the laser beam 9 in a range of, for example, about 1.5 to 1.7, having thickness of about 1.1 mm, and excellent in impact resistance, which is formed mainly of resin, for example, polycarbonate, a glass plate, an acrylic plate, or an epoxy plate is used.
The optical recording layer 3 is a layer consisting of a light-absorbing material containing dyes formed on the substrate 2. When the laser beam 9 is irradiated, heat generation, melting, sublimation, deformation, or modification is caused in the optical recording layer 3. The optical recording layer 3 is formed by uniformly coating a dye or the like dissolved by a solvent over the surface of the substrate 2 with means such as spin coating.
An arbitrary optical recording material can be adopted as a material used for the optical recording layer 3. However, the material is desirably a light-absorbing organic dye and is required to have a refractive index n exceeding 1.9 in an absorption wavelength area of the laser beam 9.
The light reflection layer 4 is a metal film with high thermal conductivity and light reflectivity. The light reflection layer 4 is formed from gold, silver, copper, aluminum, or an alloy containing these metals by means such as evaporation or sputtering.
The protective layer 5 is formed from resin excellent in impact resistance and adhesiveness, which is the same as the material used for the substrate 2. For example, the protective layer is formed by applying ultraviolet curing resin on the light reflection layer 4 with spin coating and irradiating ultraviolet rays to harden the ultraviolet curing resin.
The dummy substrate 6 is formed of the same material as the substrate 2.
As shown in, in particular, FIG. 14, a sub-information area 12 (a BCA area), a system read-in area 13, and a main information area 14 (a data area) can be defined two-dimensionally and concentrically from a center hole 11 of the optical information recording medium 1 (the substrate 2) toward an outer peripheral side.
The BCA (Burst Cutting Area) recording forms a recording area (the sub-information area 12) formed of barcodes 15 near the center of the disc (the optical information recording medium 1), and records key information and the like in the barcodes 15 using the laser beam 9 to make it possible to, for example, represent a serial number of the disk and prevent illegal copy of the disk. Sub-information is not limited to information according to the BCA. A sub-information area is not limited to the BCA area.
Note that a section of the sub-information area 12 and the main information area 14 has a structure shown in FIG. 15 and makes the barcodes 15 recordable according to absorption of the laser beam 9 by the optical recording layer 3 in the sub-information area 12.
The system read-in area 13 is located on an outer peripheral side of the sub-information area 12. The system read-in area 13 performs information recording by embossed pits.
The main information area 14 is an area that is located further on the outer peripheral side of the system read-in area 13 at a predetermined interval and used for recording and reproduction of usual optical information (data information) by the laser beam 9 used by general users. The main information is not limited to information recorded in the main information area 14 (the data area), that is, this data information recorded by the general users.
At a stage of shipment from a factory, in the optical information recording medium 1, information is written in both the sub-information area 12 and the system read-in area 13 by a writing device for BCA and an embossing device (not shown). There is a problem in that it is necessary to perform the BCA (sub-information recording), in particular, in the sub-information area 12 according to barcode recording after forming the optical recording layer 3, the light reflection layer 4, the protective layer 5, and the dummy substrate 6 on the substrate 2 independently of a stamping process for forming the pre-grooves 7 and the lands 8 in the substrate 2.
These problems are common to blue laser discs (BDs) of other types.
FIG. 16 is a schematic sectional view of an optical information recording medium 20 using the blue laser disc (in particular, BD).
As shown in the figure, the optical information recording medium 20 includes the transparent 2, the light reflection layer 4, the optical recording layer 3, the protective layer 5, an adhesive layer 21, and a cover layer 22. A plan view thereof is substantially identical with that of the optical information recording medium 1 (FIG. 14). Note that an inorganic layer may be added on the surface of the protective layer 5 or the adhesive layer 21.
The adhesive layer 21 sticks the cover layer 22 with thickness of about 0.1 mm to the protective layer 5.
When the laser beam 9 is irradiated from the cover layer 22 side, the optical recording layer 3 absorbs energy of the laser beam 9 to generate heat. Recording pits 10 is formed in the pre-grooves 7 or the lands 8 according to thermal decomposition of the optical recording layer 3 to make it possible to record information in the optical recording layer 3.
FIG. 17 is a graph (absorption spectra) of absorbance of respective discs (a CD, a DVD, a BD, and an HD-DVD) with respect to a wavelength of the laser beam 9. In addition to these spectra, a refractive index n of the optical recording layer 3 and an output for BCA (laser power) are also plotted on the graph.
Concerning the refractive index n, in all the cases of the CD, the DVD, the BD, and the HD-DVD, a maximum value of the refractive index n with respect to a laser beam wavelength of the optical recording layer 3 is located on a long wavelength side of respective absorption peaks. In order to obtain sufficient percentage modulation in optical recording (i.e., in order to set a variation An of the refractive index n large), the refractive index n is required to be large. All of the CD, the DVD, the BD, and the HD-DVD perform data logging in the main information area 14 according to absorption on the long wavelength side of the absorption peaks.
As shown in the figure, since the barcodes 15 have a large area compared with the recording pits 10, in order to write the barcodes 15 for BCA, a laser with power (about 1000 mW) larger than laser power (about 100 mW) for usual data writing in the main information area 14 is required. However, as an oscillation source (semiconductor) for a blue laser for a BD and an HD-DVD, an oscillation source having sufficient power has not be developed yet (maximum power is about 200 mW at present). Thus, in order to perform the BCA, an oscillation source of a red laser beam already used for a CD or a DVD (a red laser beam for a CD or a short wavelength red laser beam for a DVD, hereinafter referred to as “red laser”) is used under the present circumstances.
Therefore, in the identical disc (the optical information recording medium 1 (FIG. 15) or the optical information recording medium 20 (FIG. 16)), wavelengths of oscillation lasers using semiconductors are different in the barcode recording (the sub-information recording) in the sub-information area 12 and the data logging (main information recording) in the main information area 14. Thus, there is a problem in that it is necessary to contrive characteristics of the optical recording layer 3 of the optical information recording medium 1 or 20.